The present invention relates to the field of electromagnetic and seismic monitoring of microseismic events. In particular, the invention relates to a method and system for monitoring both electromagnetic and seismic energy from microseismic events and using the data to more accurately determine the time, location, and other characteristics of the event.
During oil, gas or groundwater production it is often the case that fluid either injected or removed from the rock formation causes chemical, temperature or pressure changes that in turn cause the rock to fracture or fault. Fracturing or faulting emits microseismic energy, the hypocentre of which is known as a microseismic event. In general, a microseismic event is triggered by any fracturing or faulting in the rock regardless of the cause. For example, microseismic events can also be caused by mining, which alters the stress field in the Earth by removing material that would otherwise support stress. Microseismic events produce seismic waves that propagate through the surrounding rock. These seismic waves, in turn, can often be detected by particle motion detection devices on the ground surface, in water, or in boreholes.
Detecting, locating and characterizing induced microseismic events provides potentially valuable information about the structure of the subsurface, especially about the paths along which fluid is flowing. In the past, these microseismic events have been located and characterized by inverting various attributes of the seismic energy released (e.g., arrival time, phase, polarization).
However, using seismic data alone is severely deficient for characterizing microseismic fracture events for two main reasons: First, the time taken for seismic energy to propagate between the event and the particle motion sensor is appreciable and is medium-dependent. This implies that relocating events accurately depends entirely on having an excellent Earth model for seismic energy propagation. This is often not the case for compressional P wave data and is seldom the case for shear S wave data. Second, even if full waveform data is used, significant trade-offs exist between the various parameters that describe the source mechanism. An important parameter with which many others trade-off is the microseismic event source time.
It has been known for some time that electromagnetic energy could be used for seismic imaging. For example, some applications detect electromagnetic energy that comes from electrolytic fluids in the earth interacting with man-made seismic waves (see e.g., Geophysics 62, No. 1, 1997, pp. 97-105 xe2x80x9cElectric Investigation of the Shallow Subsurface: Field Measurements and Numerical Modelingxe2x80x9d Mikhailov, Haarsten and Toksoz.) Additionally, other applications have used man-made electromagnetic energy sources to aid in subsurface imaging (see e.g., U.S. Pat. No. 3,975,674). Still other applications have attempted to use electromagnetic energy to predict earthquakes (see e.g., U.S. Pat. No. 5,694,129 to Fujinawa et al.).
However, no effective method has been proposed for locating or characterizing microseismic events that effectively incorporates the measurement of electromagnetic and seismic energy caused by the microseismic event itself.
Another related problem is that of providing an effective triggering mechanism for remote seismic sensors. Methods of using the seismic signal itself to trigger remote seismic measurement have the disadvantage of not consistently recording the onset of seismic energy.
Thus, it is an object of the present invention to provide an effective system and method of reducing uncertainty in relocation and characterization of microseismic events though measurement of both electromagnetic and seismic energy caused by the microseismic event. It has been found that as the microseismic event occurs, a piezo-electric potential is created instantaneously as the sides of the fault or fracture move relative to each other; electrolytic fluid (often water) may also be xe2x80x9csquirtedxe2x80x9d along the fracture or fault plane. Both of these processes produce electromagnetic (EM) signals that travel at great speeds through the surrounding rock and hence may be detected almost instantaneously by remote EM receivers in boreholes, on the ground surface, in water, or on the sea floor.
The virtually instantaneous arrival time of the electromagnetic energy greatly alleviates the fundamental trade-off problem between event time and location. Additionally, it has been found that the arrival time of the electromagnetic energy can also be used to trigger recording of the approaching seismic energy.
Advantageously, according to the invention, the travel times of all arriving phases of the seismic energy (e.g., P-waves, S-waves, and surface waves) can be determined very much more accurately and independently (since the source time is known). This, in turn, provides information that can be used to constrain the seismic velocity structure of the Earth. Additionally, the phase and vectorial components of the electromagnetic signal contain information about the character and direction of the seismic event relative to the recording sensor. It has also been found that the amplitude of arriving electromagnetic energy contains information about the conductivity of rock between the event and receiver.
According to a preferred embodiment, the equipment used to record the electromagnetic signal can be an antenna or electrode, either of which can be embodied in a resistivity array with switchable sensitivities and bandwidth. When co-located with ground motion sensors, the detection of electromagnetic source signatures can be used to trigger recording of approaching seismic energy. This can also be used to trigger recording when man-made seismic sources are activated on the surface or in remote boreholes with no real-time connection to the receivers. The equipment may be either transitory or located permanently.
According to the invention a method of determining the location of a microseismic event is provided. An electromagnetic sensor measures the electromagnetic energy, at least some of which is caused by a microseismic event that consists primarily of rock fracturing or activation of existing rock fractures. Seismic data from the microseismic event is measured and recorded using plurality of ground motion sensors. The time at which microseismic event occurred is estimated based on the time at which electromagnetic energy caused by the microseismic event is received by the electromagnetic sensor. The location of said source of the microseismic event is determined using the estimated time at which the microseismic event occurred and the time at which the earth vibrations caused by the microseismic event arrived at the ground motion sensors.
According to an alternative embodiment, a method of determining the time of occurrence of a microseismic event and triggering a seismic recording device is provided. The electromagnetic energy from a microseismic event is measured and recorded using an electromagnetic sensor. The time at which the source of the seismic energy occurred is estimated based on the time at which electromagnetic energy caused by the source of seismic energy is received by the electromagnetic sensor. Recording by a seismic recording device is then initiated in response to the estimated time at which the source of seismic energy occurred.
The present invention is also embodied in a system for determining the location of a microseismic event, and a computer readable medium that carries instructions to direct an apparatus to determine the location of a microseismic event.